US officials on Monday said the results of a UN report on chemical weapons use in Syria, released the same day, clearly show that the government of Bashar Assad was behind the August 21 attack on a Damascus suburb in which more than 1,400 people were killed, according to US figures.

In releasing their report, UN inspectors said Monday there is “clear and convincing evidence” that chemical weapons were used on a relatively large scale in the attack last month in Syria. The findings (click here to read the full report) represent the first official confirmation by scientific experts that chemical weapons were used in Syria’s civil war, but the report left the key question of who launched the attack unanswered.

In a press conference, US Ambassador to the United Nations Samantha Power said that “the technical details of the UN report make clear that only the regime could have carried out this large-scale chemical weapons attack.”

One type of munition, 122mm rockets, were found to have been used in the attack, a type associated with “previous regime attacks,” Power said, adding that “we have not observed the opposition manufacturing or using this style of rocket.”

In addition, Power said, lead investigator Ake Sellstrom, in presenting the report, said that the quality of sarin used in the attack was “higher than the quality of that used in Saddam Hussein’s program” and that weapons found at the site were “professionally made” and “bore none of the characteristics of improvised weapons.”

“It’s very important to note that the regime possesses sarin, and we have no evidence that the opposition possesses sarin,” Power said, adding that “in the days before the attack, Assad’s chemical weapons experts prepared for an attack. They distributed gas masks to regime troops. They fired rockets from a regime-controlled area into 12 neighborhoods that the regime had been trying to clear of opposition forces.”

The August 21 attack, Power noted, was “the largest chemical weapons attack in 25 years.”

In a Monday statement, President Barack Obama’s security adviser Susan Rice said that the evidence presented, particularly the type of rocket used and the high-quality of the sarin, ”reinforces our assessment that these attacks were carried out by the Syrian regime, as only they had the capability to mount an attack in this manner.”

The rebels and their US and Western supporters have said the regime of President Bashar Assad was behind the August 21 attack, while the Syrian government and its closest ally, Russia, blame the rebels.

US, British and French diplomats said the findings of the UN inspectors supported their conclusion that the Assad regime was to blame.

Secretary of State John Kerry briefed US allies on a broad agreement reached over the weekend with Russia to end Syria’s chemical weapons program, pressing for broad support for the plan that averted a US military strike. Kerry met in Paris with his counterparts from France, Britain, Turkey and Saudi Arabia before seeking a UN resolution that would detail how the international community can secure and destroy Syria’s stockpile and precursor chemicals.

The Syrian National Coalition — the main umbrella opposition group — welcomed the inspectors’ report and urged the Security Council to hold the Assad regime responsible for the attack and refer the Syrian government to the International Criminal Court for prosecution.

General Salim Idris, head of the rebel Free Syrian Army, said in an interview on the PBS NewsHour that the inspectors’ report makes “very clear that there’s a war crime.” He said the Syrian people “are very frustrated because of what’s going on and because the international community is not caring anymore about the victims.”